In the field of telecommunications, efforts have recently been directed toward developing advanced direct sequence spread spectrum (DS-SS) telecommunications systems. One example of a DS-SS type system is a Code Division Multiple Access (CDMA) type system.
In a CDMA type system multiple users, each using a channel identified by a uniquely assigned digital pseudonoise (PN) code sequence, simultaneously communicate with the system while sharing the same wideband frequency spectrum. Channel identification is achieved by using the unique PN code sequence to spread a digital information signal that is to be transmitted. The digital information signal may be a signal, such as the output of a digitized voice circuit having a bit rate, for example, of from 8-13 kbps, or a data signal having a higher bit rate. The PN code sequence has a bit rate greater than the bit rate of the information signal.
During spreading, the digital signal bandwidth is spread through the frequency bandwidth of the PN code sequence. Spreading is achieved by multiplying the PN code sequence and information signal together in the time domain to generate a spread signal that has a bit rate of the PN code sequence. The spread signal is then RF modulated and transmitted on a carrier frequency that may also carry transmissions of information signals for other system users, where the other information signals have been spread by PN code sequences unique to each of the other users. The PN code sequences may be identified by having a unique phase or a unique bit sequence. In certain systems, such as a system operating according to the Telecommunications Industry Association/Electronic Industry Association (TIA/EIA) IS-95-A cellular system standard, a transmission may be identified by two PN sequences. In IS-95-A an individual base station is assigned a unique phase for a common system PN code sequence that spreads all forward link transmissions from base stations of that system. The unique phase identifies the base station. Each transmission from a base station is then assigned a unique Walsh code sequence that identifies the particular base station channel on which the transmission is sent.
At the receiver, after carrier frequency demodulation, despreading or correlation is accomplished by generating a local replica of the transmitting user's assigned PN code(s) with a random-sequence generator in the receiver and then synchronizing the local PN code sequence(s) to the PN code sequence(s) that was superimposed on the incoming received signal in the transmitter. By removing the random sequence from the received signal and integrating it over a symbol period, a despread signal is obtained which ideally exactly represents the original digital information signal.
Because data transmission applications other than conventional voice traffic transmissions are becoming increasingly important in the cellular system area, a system operator who operates a CDMA system may desire to provide services other than phone voice service. Examples of these other services include portable computer modem service or video service. Often, these other services may require that data be transmitted at a rate much faster than that required for voice transmission.
Developments in the cellular industry have moved in the direction of satisfying requirements for higher data rate cellular service. For example, a new version of the IS-95-A specification to be published as TIA/EIA IS-95 includes supplemental channels. Up to seven supplemental channels in addition to a fundamental channel may be added for use between a mobile station and base station, allowing a total of eight channels for data transmission on each of the forward (base-to-mobile) and reverse (mobile-to-base) links. Data can be transmitted at rates of up to 115.2 kbps (a maximum of 14.4 kbps for each channel) if all seven supplemental channels are utilized.
For the next generation CDMA cellular systems, it has been proposed to further increase the data transmission rate above that available in IS-95 by using variable length Walsh codes on the forward link channels. In schemes of this type, a constant chip rate is used for the Walsh codes but the length of the Walsh code varies according to the data rate. For example, this type of scheme could be implemented in a system similar to an IS-95 system by using the orthogonal standard 64-bit Walsh code at a chip rate of 1.22 mcps for 9.6 kbps transmission and using shorter lengths of the Walsh code at the same chip rate for other data rates. For 19.2 kbps a 32-bit Walsh code at a chip rate of 1.22 mcps could be used. At 19.2 kbps the spreading factor would be halved as compared to 9.6 kbps, i.e., each bit of data would be spread (multiplied) by half as many chips. The data frame could be a constant 20 msec. in length with the amount of data contained in a frame proportional to the data rate.
The next generation high data rate of CDMA cellular systems will be required to offer both conventional services, such as voice service, and packet data services. Efficient utilization of the variable length Walsh codes is required, because the required orthogonality limits the number of codes from which to choose. For example, in a CDMA system where the chip rate is three times the IS-95A chip rate, the Walsh code will be assignable from a Walsh code space of 256 codes of 256 bits in length. The 256 codes allow capacity to carry a total of 256.times.14.4 kbps of traffic per call using 1/2-rate convolution coding. Although this is greater than three times what is needed for the theoretical upper limit of the spectrum efficiency, due to interference reasons, these code are still reasonably limited. It would be inefficient to preassign these codes to a large group of packet users who only use traffic channels in a burst mode when the bursts are infrequent. Use of the system requires dynamic assignment of the Walsh codes at the time when a traffic channel is actually required.
In order to achieve efficient packet data routing in next generation systems, it will be necessary to assign and reassign traffic channels very frequently among numerous users. The dynamic nature of packet data traffic requires that as soon as one packet data service user finishes a channel, it can be assigned quickly to another packet data service user. Because each multi-rate traffic channel is spread by a Walsh code having a length corresponding to the data rate on the channel, Walsh code index and length information must be sent to each packet data user before the channel can be used. The use of supplemental channels to realize multiple traffic channels combined with the use of variable length Walsh codes increases the signaling overhead needed to assign the Walsh codes to a mobile station. This signaling overhead can be excessive if the code index is large and particularly if multiple codes need to be assigned. In order to realize an efficient system, it is desirable that this overhead be minimized.